The visual information is recognized when the light that entered from the cornea, which is a transparent tissue on the frontmost surface of the eyeball, reaches the retina to excite the retinal nerve cell, and the developed electric signals reach the visual field in the cerebrum via optic nerve. In other words, the cornea is positioned in front of a path where the light passes when a living organism receives visual information. Therefore, the turbidity in the cornea caused by damage and the like has a serious influence on the visual function.
Histologically, the cornea has a three-layer structure of corneal epithelium, corneal stroma, and corneal endothelium from the outer surface side. The transparency of cornea is maintained since the water content is kept constant by Na+ active transport (pump function) by Na, K-ATPase and barrier function (tight junction protein such as ZO-1 and the like) in the corneal endothelium.
The above-mentioned function of corneal endothelial cell is impaired by a damage to the corneal endothelium such as a decrease in the corneal endothelial cell and the like, thus resulting in the edema of the corneal stroma. This decreases transparency of the cornea, and reduces the eyesight. Such condition is called bullous keratopathy. In the meantime, it is known that human corneal endothelial cell once injured scarcely shows an ability to regenerate. When the corneal endothelial cells have decreased due to certain injury, an effective or, in some cases, sole treatment thereof is corneal transplantation. In fact, about half the number of applicable cases of corneal transplantation is for bullous keratopathy caused by corneal endothelial functional disorder.
At present, patients with corneal endothelium damage are treated by penetrating keratoplasty wherein the whole three-layer structure of corneal epithelium, corneal stroma and corneal endothelium is transplanted. While the penetrating keratoplasty is an established technique, the supply of cornea is short in Japan as the situation stands, and the rejection reaction poses a problem. To solve such problems, “part transplantation” involving transplantation of only the damaged tissue is becoming popular. Deep lamellar keratoplasty (DLKP) involving transplantation of only the epithelium and stroma of the donor while preserving corneal endothelium (non-patent documents 1 and 2), corneal endothelium transplantation involving transplantation of only the part cornea including endothelium (patent documents 1 and 2, non-patent document 3) and the like are known. However, in the case of corneal endothelium transplantation, for example, the source of supply of the material for, transplantation is still the corneal endothelium itself. Since the number of donor of cornea is limited, the problem of donor shortage cannot be overcome, like penetrating keratoplasty. Furthermore, since corneal endothelial cell is difficult to culture, preparation of cultured cells in a number sufficient for transplantation places a large burden in terms of time and cost.
In recent years, separation of cells, having characteristics of stem cell, from the mouse corneal stroma has been reported (non-patent document 4). This stem cell derived from the neural crest has an ability to also differentiate into a nerve cell, an adipocyte and the like by using a differentiation induction medium.
While it has been reported that adhesive culture of tissue stem cell/progenitor cell in a medium added with TGFb2 can induce differentiation of these cells into corneal endothelial cells (patent document 3), the presence or absence of the function as corneal endothelial cell (for example, pump function) is not shown.